Automatic telephone exchanges comprising digital time-switching networks for the switching of telephone channels carried in time division multiplexes employing pulse code modulation (PCM) for example, are known. In a certain digital PCM transmission system, signalling is transmitted in predetermined time-slots of the time division multiplex, being injected and removed by peripheral units linked to the central computer.
The switching network is controlled by the computer through a marking peripheral. Regarding injection and extraction of signaling information, the appearance of the digital switching network has made it more economic to use said network itself, doing away with the testers and distributers previously carrying out these functions in the line equipment groups or preselection stage. The contents of the signalling time-slots of incoming and outgoing PCM systems are switched on digital PCM systems assigned to signalling peripherals. Exchanges of this type are described in the journal "COMMUTATION ET ELECTRONIQUE" No. 56 of January 1977, pages 39 to 43, FIG. 1, and in No. 59 of October 1977, pages 35 and 36, FIG. 4.
This type of structure has a disadvantage, namely the cost of linking the signalling peripherals to the computer. Indeed, to retain the advantages of central common control, peripherals may be used which do not analyse signalling information, but simply react to such basic signals as leading pulse edges and status changes. The flows of messages between these peripherals and the computer are then very heavy, and substantial means are needed for their control. An example of such links between peripherals and their computers is described in French Pat. No. 2.346.930 of Mar. 31, 1976, to the present applicant. This type of link demands logical processing facilities at each end (module interfaces CM and concentration processing circuits LC).
Further, the cost of such links can increase substantially with their length.
The purpose of the invention is to do away with these disadvantages by making greater use of the digital PCM channels switched through the switching network.